1. Field of the Invention
The present invention relates in general to a control circuit of power converter, and more particularly, to a control circuit for synchronous rectifying and/or soft switching of power converters.
2. Description of Related Art
FIG. 1 shows a circuit schematic of a soft switching power converter, such as a resonant power converter, which includes a power transformer 10 to provide isolation from AC line input to the output of the power converter for safety. The soft switching of the power converter achieves high efficiency and low EMI (electric-magnetic interference) performance. Transistors 20 and 30 develop a half bridge circuit to switch a resonant tank. The resonant tank is formed by the power transformer 10, an inductor 15 and a capacitor 40. The inductor 15 acts as a primary-side leakage inductance of the power transformer 10 and/or an inductance device. The inductance L of the inductor 15 and the capacitance C of the capacitor 40 determine the resonance frequency f0.
                              f          0                =                  1                      2            ⁢            π            ⁢                                          L                ×                C                                                                        (        1        )            
The power transformer 10 transfers the energy from the primary side to the secondary side of the power transformer 10. Rectifiers 41 and 42 rectify the switching voltage of the power transformer 10 into a capacitor 65. A DC voltage VO is thus outputted at the output terminal of the power converter.
In recent development, applying the synchronous rectifier on the secondary side of the power transformer is a further approach to achieve a higher efficiency power conversion, such as “Control circuit associated with saturable inductor operated as synchronous rectifier forward power converter” by Yang, U.S. Pat. No. 7,173,835. However, the disadvantage of this prior art is an additional power consumptions caused by saturable inductors, etc.